PROJECT SUMMARY Mosquito arbovirus transmission causes hundreds of millions of human infections every with significant disease morbidity and mortality outcomes. However, despite the requirements of the mosquito host for virus replication and transmission, our understanding of these mechanisms is poorly understood. This includes the role of mosquito immune cells, or hemocytes, which may have both positive and negative influences on virus replication. Building on our expertise in hemocyte biology and host-pathogen interactions, we now look to examine the role of mosquito hemocytes on dengue (DENV) and Zika virus (ZIKV) infection. Using a chemical genetics approach, we provide preliminary data demonstrating our ability to deplete phagocytic immune cell populations in Aedes aegypti, which serve as the basis of our proposed experiments. Therefore, the studies in this proposal will examine the contributions of phagocytic immune cells on DENV and ZIKV infections, and explore the mechanisms by which these cells may mediate systemic RNAi responses in the mosquito host (Aim 1). In addition, we will also explore potential roles of hemocytes as a secondary site of virus amplification that may increase viral titers and aid dissemination to increase the likelihood of transmission (Aim 2). Together, we expect that the results of this study will provide new information into the mechanisms of virus replication in the mosquito host and define the integral role of hemocytes in mosquito vector competence to arbovirus infection.